How to Apply Self-Leveling Underlayment on Plywood Subfloors
- Isaac Ostrom
- 12 hours ago
- 10 min read
Alright, here’s the deal. Foam shower pans are awesome when the floor is actually level. But if your plywood subfloor is out of whack even a little bit, that foam pan is going to follow whatever the floor is doing. And then you’ve got water trying to run somewhere other than the drain, and you’re standing there wondering why your “pre-sloped pan” is acting like a bird bath.
So in this one, I’m putting self-leveling underlayment (SLU) over a plywood subfloor because I’m installing an RSS (Revolutionary Shower Systems) foam pan. I’m not sponsored by RSS. I just like supporting smaller companies that are doing good work. The big point today is this: self-leveling on plywood is not the same as self-leveling on a concrete slab, and if you treat it the same, you can make a mess real fast.
I’m going to walk you through exactly how I prep, dam, prime, staple lath, calculate how many bags I need, mix it, and pour it. I’ll also tell you what I’m watching for so the leveler doesn’t disappear under a wall or run into the next room like soup.
Key Takeaways for Homeowners & Pros [TLDR Version]
Foam pans need a level base. If the subfloor slopes or has a crown, the pan won’t drain the way it’s designed to.
On plywood, you need to control where the SLU can go. If you don’t dam the perimeter and holes, it’ll run like water and disappear into every void it can find.
Duct tape plus expandable foam is a simple way to seal the perimeter, seams, drain hole, and any gaps along the bottom plate.
Prime the plywood before pouring. That’s what helps the SLU bond instead of separating or getting weird.
On plywood, galvanized lath is the move. Staple it down tight so the SLU has mechanical reinforcement and the floor stays stable.
Figure out your high spot first. Then measure the low spots so you can estimate average thickness and bag count before you mix.
Pre-measure your water and have your buckets ready. Once you start mixing, you’re on the clock.
Start pouring at the deepest area and let it seek its level. Use a flat trowel to help it along, especially where it gets thin.
Bucket liners are underrated. They keep your measurements consistent and cleanup is way easier.
Tools Required
Here’s what I used on this job:
Self-leveling underlayment: Custom Building Products LevelQuik (easy to find at Home Depot or a tile supplier)
Primer (for plywood)
Galvanized lath sheets (2.5 ft x 8 ft)
Duct tape (for perimeter and seams)
Expandable foam in a can (for dams at holes, voids, and gaps)
Angle grinder with a diamond wheel (to cut lath)
Staple gun: DuoFast CS5000 (or similar)
3/8" galvanized staples (I stapled about every 6 inches)
4-foot level
Shims (for measuring how far out of level the floor is)
Buckets (at least two if you’re mixing two bags)
Drill and paddle mixer (egg beater style works too)
Flat trowel (I used a Marshalltown light trowel)
Bucket liner with measurement lines (bucket saver style)
Why I’m Leveling a Plywood Subfloor Before a Foam Pan Goes Down
This is the part a lot of people don’t think through.
A foam pan is manufactured to slope to the drain. But it’s not magic. If your subfloor is sloped a half inch over four feet, the pan is now riding on that slope. Water will take the easiest path, and sometimes that path is toward the wall or toward the curb instead of into the drain. That’s not a “foam pan problem.” That’s a “base prep problem.”
And on this particular floor, it wasn’t just a gentle slope. There was also a crown because a girder runs through the floor. So the level bubble was doing different things depending on which side of that crown I was measuring. That kind of floor will fight you if you skip leveling.
So my mindset is simple: if I’m using a foam pan, I just assume I’m going to level the subfloor. Sometimes you get lucky and it’s already flat. I rarely see that.
Step 1: Seal the Perimeter Like You Actually Want to Keep the SLU in the Room
Before I even think about primer or lath, I’m thinking containment.
Self-leveler doesn’t “kinda flow.” It runs like water. If there’s a path out, it’s taking it. If there’s a gap under a bottom plate, it’ll run under it. If there’s a seam between plywood sheets, it’ll find it. If there’s a drain hole, it’ll head straight for it.
So the first thing I did was take duct tape and seal the perimeter. In this bathroom, the tile was already installed outside the shower footprint, and it was about three-quarters of an inch thick. That tile acted like a natural dam. If the tile wasn’t there, I’d need to build a dam with foam or something else so the SLU doesn’t just run out into the rest of the floor.
I ran duct tape around the whole perimeter and I paid special attention to the bottom plate sitting on the subfloor. I do not want self-leveler running under that plate. Once it gets under there, you’re not “cleaning it up.” You’re chiseling it out later and being mad at yourself.
Then I used expandable foam (the canned stuff) to create dams where I needed them:
Around the drain hole
At any voids along the bottom plate
Anywhere big enough that the leveler could dump into it and disappear
Over seams that looked like they might leak
And you don’t need to overthink the foam. The goal is just to block the pathways so the SLU stays where you need it.
One quick timing tip: the foam I used was tack-free in about 20 minutes. By the time I was pouring, it was already hardened enough to do its job.
Step 2: Prime the Plywood
Once the perimeter and holes are sealed, I prime the plywood.
Primer isn’t optional if you want this to bond. Plywood is thirsty. If you dump SLU straight on raw plywood, it can suck the moisture out of the mix, mess with flow, and hurt bond. Primer is what gives the SLU a fighting chance to grab and cure the way it’s supposed to.
I primed the entire surface after I had my duct tape and foam dams in place.
Step 3: Install Galvanized Lath and Staple It Down Tight
After the primer, I stapled down galvanized lath.
The sheets I used are the common 2.5 ft by 8 ft pieces you can get at Home Depot or Lowe’s. I cut them to size and fitted them into the area.
How I cut lath without hating my life:
Angle grinder
Diamond wheel (just leave it on)
Cuts clean and fast
Then I fastened it down using my DuoFast stapler (CS5000). I went about every 6 inches with 3/8" galvanized staples.
This step matters. If your lath is loose, it can lift, flex, or create weird spots in the pour. You want it seated and locked down so the SLU and lath act like one reinforced layer.
I use that stapler for a bunch of stuff, by the way. Hanging wire for floated walls, stapling lath, whatever. It’s one of those tools that pays for itself if you do this kind of work.
Step 4: Find the High Spot and Calculate How Much SLU You Need
This is where people either look like pros or look like they’re guessing.
You need to know how many bags you’re going to use before you start mixing. Because once you start, you don’t want to be halfway through a pour and realize you’re short. And you also don’t want to mix too much and overflow your dam or bury yourself.
So here’s what I did.
First, I found the high spot in the floor. I put my 4-foot level right on the lath, no shims, just to see what the floor was doing. Then I used shims under the low side until the level read true.
That shim stack tells you how far out of level you are between those points.
On my high spot, I was around 7/16" out in one measurement direction, and the slope continued more in another direction. Again, there was also a crown in the floor because of the girder. The point is: it wasn’t close.
Now, with SLU over lath, I want a minimum thickness that actually covers the lath. The lath itself is about 1/8". I want at least 1/4" at my high point so I’m fully buried over the mesh and not starving it.
Then I estimate what the low point will end up needing to be. For example, if I’m targeting 1/4" at my highest point, and I need roughly another 1/2" to bring the low areas up, now the low areas might need around 3/4".
So I’m thinking:
High point thickness target: 1/4"
Low point thickness target: about 3/4"
Average thickness: about 1/2"
That average thickness is what helps me estimate bag count.
In my case, this was about a 20 square foot area, and based on coverage rates, I figured two bags should get me about a half inch average thickness. I also knew I might not dump every last drop, because I’d rather stop a little short than overflow and make a bigger problem.
This is one of those “pay attention now so you don’t pay for it later” steps.
Step 5: Pre-Measure Water, Stage Buckets, and Mix Like You Mean It
Once I know my bag count, I stage everything.
I like having two buckets of water ready to go before I start mixing. That way, I’m not scrambling mid-mix. I’m moving efficiently because SLU doesn’t wait for you.
For LevelQuik, I mixed at 5.5 quarts of water per bag. I like a bucket liner that has measurement lines because it makes this simple. I’ve also used paint buckets with measurement marks before, but the bucket liner is just slick. You drop it in, measure, mix, and cleanup is easy.
For mixing:
Pour the powder in slowly (if you dump fast, it can puff and spill)
Use a drill and paddle
Mix thoroughly, all the way to the bottom
Then right before you pour, give it another quick spin
That second spin matters more than people think. If it sat for a minute while you mixed your second bucket, it can start to thicken slightly. A quick remix makes it pour smoother and keeps the flow consistent.
Step 6: Pour at the Deepest Point and Help It Find Its Level
Pouring is the fun part, but it’s also where you can make a mess fast if you didn’t prep right.
My approach:
Start where it’s deepest
Dump the first bucket
Let it flow
Use a flat trowel to guide it, especially into thinner areas
I used a Marshalltown light flat trowel. People use spike rollers and gauge rakes sometimes, and those tools are great for bigger pours. But for a smaller footprint like this, the flat trowel is enough to help it move where it needs to go.
And here’s something I’ve noticed: SLU finds its level better when it has enough thickness to flow. Where guys struggle is the thin edges. On concrete, surface tension can hold it back. On plywood with lath, you can still get thin spots that need a little encouragement. That’s why I don’t just dump and walk away. I watch it, and I help it.
My dams worked great. The expandable foam did its job, and the duct tape kept things tight at the perimeter.
I poured the first bucket fully, then I poured about half of the second bucket, because I didn’t want to overflow. I ended up with about a quarter bucket left, and that was perfect for this situation.
After a little working and watching it settle, I could tell I had what I needed:
About 1/4" at the high spot
A nice smooth transition into the low areas
A surface that’s going to be level enough to accept the foam pan the way it’s meant to sit
Step 7: Cleanup and the Bucket Liner Trick
Cleanup is where you either feel like you had a good day or you feel like you got punished.
This is where the bucket liner is money. I had leftover SLU in the liner, and instead of stressing about dumping it somewhere, I can just let it harden and pop it out later. That’s a win. Especially if it’s a Friday and you’re trying to wrap up clean and not ruin your buckets.
Pro Tips I’d Tell Anyone Doing This for the First Time
Treat every edge like it’s going to leak. Because if it can leak, it will.
Seal the bottom plate. Self-leveler under a wall is not a fun surprise.
Don’t skip primer. If you want the SLU to bond, prime the plywood.
Staple lath tight and consistent. Loose lath makes bad pours.
Find the high spot first. Your whole thickness plan is based on that.
Mix accurately. Measure water. Don’t guess.
Have everything ready before you mix. Buckets, drill, trowel, pathways clear.
Start at the deepest area and work outward.
Help it in thin zones. A flat trowel is your steering wheel.
Don’t overpour just because you mixed it. Stop when you hit your target.
FAQ Section
Q. Do I really need SLU under a foam shower pan?
If your subfloor isn’t level, yes. A foam pan will follow the floor. If the floor slopes or crowns, water can run the wrong direction instead of toward the drain.
Q. Can I pour self-leveler directly onto plywood?
Not the right way. Plywood needs primer, and in most cases you’ll use lath for reinforcement and stability. Plywood moves differently than concrete, and you have to treat it accordingly.
Q. Why do I need to seal the perimeter and holes?
Because SLU runs like water. If there’s a gap under the wall, a seam, or a drain hole, it will flow into it and disappear. Duct tape and expandable foam are simple ways to dam it.
Q. How do I know how many bags I need?
Find your high spot, measure the low areas with a level and shims, decide your minimum thickness at the high spot (enough to cover lath), then estimate the low spot thickness and average it. That average thickness helps you estimate bag count for the square footage.
Q. What’s the best way to cut galvanized lath?
Angle grinder with a diamond wheel. It’s quick and cuts clean.
Q. How long do I wait before installing the foam pan?
Follow the SLU manufacturer’s cure times and conditions. The key is you want it set and stable before you start bonding a foam pan to it.
Q. Do I need a spike roller?
Not always. For small pours, you can usually guide it with a flat trowel. Spike rollers and gauge rakes are great for bigger pours, but you don’t have to overcomplicate a small footprint.
Q. What’s the biggest mistake people make with self-leveler on plywood?
Skipping prep. If you don’t dam it, prime it, and reinforce it, you’re asking for leaks, separation, cracking, or a pour that doesn’t end up level.
Final Thoughts
If you’re installing a foam pan and you want it to work like it’s supposed to work, you’ve got to give it a fair base. I don’t care what brand pan you’re using. If the subfloor is crowned, sloped, or just generally doing its own thing, the pan will follow it. That’s how you end up with water running away from the drain and a shower that never feels right.
The good news is this isn’t complicated. It’s just detailed.
Seal the perimeter. Foam the voids. Prime the plywood. Staple the lath tight. Measure the floor like you mean it. Mix accurately. Pour smart. Help it in the thin zones. And don’t get greedy with the last half bucket if you’re already where you need to be.
If you’re working through a project and you want another set of eyes on it, I do one-on-one coaching sessions (30, 45, and 60 minutes) at tilecoach.com. Sometimes all you need is somebody to help you think through the order of operations before you start dumping product.
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